Zillow is not engaged by a client, it is engaged by an unknown person from the public, clicking for an instant "zestimate" . ( similar to AVM I presume) Zillow can call it an "appraisal," but clearly anyone relying on it as they would a "real" appraisal is an idiot....yet it is the public's right to be idiots and rely on a free click service if they wish to.
Zillow makes its money from advertising and RE related such as listings, RE agents signing up for leads etc. A free "appraisal" is a gimmick to draw people to the site and get them clicking to bolster ad rates and feed leads to agents etc.